1. Field of the Invention
The present invention relates to a copying apparatus or a laser beam printer which uses electrophotographic techniques.
2. Related Background Art
Referring to FIG. 2 which illustrates a printer, an electrophotographic photosensitive drum 1 as an image bearing member rotates clockwise. A corona charger 2 for evenly charging the surface of the photosensitive drum 1 is constituted by a wire and grid. A charge removal lamp 8 discharges the surface potential of the photosensitive drum 1 to close to 0 V before charging is performed by the primary charger 2. An exposing system 3 consists of a laser beam source, a collimator lens, and a polygon mirror. A developing unit 4 forms a visible image from a latent image formed on the photosensitive drum 1. A cleaner 5 recovers toner remaining on the surface of the photosensitive drum 1.
A transfer sheet feed device 6 transfers the toner image formed on the photosensitive drum 1 onto a recording medium P while feeding the recording medium P held by a drum-like transfer sheet 6a. A transfer charger 7 is also provided. A toner density sensor 10 senses the mixing ratio of the toner and carrier for image formation contained in the developing unit 4. A surface potential detector 11 is means for detecting the surface condition of the photosensitive drum 1. A CCD sensor 14 reads a test patch 22; the test patch 22 illuminated by patch illuminating light sources 12a and 12b is imaged on the CCD sensor 14 through an optical lens 13. A density converter 16 converts an output voltage obtained by the CCD sensor 14 into a density. A central arithmetic circuit 100 manages each detection information described above to control image formation conditions.
FIGS. 3A and 3B are enlarged views showing the exposing system 3, in which a laser unit 31 constituted by, e.g., a semiconductor laser beam source and a collimator lens, and a polygon mirror 32 are illustrated.
FIG. 4 shows a reader (original reading device), in which an original 51, a halogen lamp 52 as a light source for illuminating an original, a mirror 53, a lens 54, a CCD 55, and an image processing unit 56 are illustrated.
Image formation processing will be described below in the order of processing steps.
First, in FIG. 4, the original 51 is illuminated by the halogen lamp 52 to form reflected light proportional to the density of the original image. This reflected light is imaged on the CCD 55 via the mirror 53 and the lens 54 and sequentially decomposed into pixels and at the same time converted into an electrical signal of high or low level. This electrical signal is subjected to magnification changing processing, moving processing, and the like performed by the image processing unit 56 and then supplied to the printer unit.
Subsequently, in FIG. 2, the photosensitive drum 1 is evenly charged by the primary charger 2 and exposed to a laser beam from the optical system 3. On the basis of the image-processed signal from the reader, as shown in FIGS. 3A and 3B, this laser beam forms a potential latent image in the thrust direction of the photosensitive drum 1 through the rotation of the polygon mirror 32 and in the circumferential direction of the photosensitive drum 1 through the clockwise rotation of the photosensitive drum 1. This latent image is developed into a visible toner image by the developing unit 4. The toner image is transferred onto the recording medium P by the transfer charger 7. The above processing steps are executed for each of magenta, cyan, yellow, and black while the developing unit 4 rotates or operates in sequence in the case of a system in which the four colors are fixed. The development results of the four colors are fed to a fixing device (not shown) to yield a full-color image.
The central arithmetic circuit 100, on the other hand, designates an output of a test patch to be detected at a predetermined time interval, forming the test patch on the photosensitive drum in the same manner as described above. This test patch consists of one or a plurality of pattern patches each having a predetermined density. The detected toner density is arithmetically processed by the central arithmetic circuit 100, thereby controlling the image forming means (e.g., the charge potential, the LUT, the toner density, and the transfer current).
FIG. 5 is an enlarged view showing the test patch 22 after being developed.
In copying machines capable of reproducing a continuous gradient, a known technique of PWM (Pulse Width Modulation) is extensively used as means capable of expressing the continuous gradient. This PWM is a method of synthesizing a triangular wave 23 with a period of 200 lpi and an image signal 24 formed on the basis of an image signal, as shown in FIG. 6A, thereby obtaining a superposed portion 25 of the two waves as an image recording signal, as shown in FIG. 6B. As a result, vertical stripes of 200 lpi typical of the rectangular wave are also found in the resulting image.
The reading circuit of the CCD 14 will be described next.
The CCD sensor 14 is driven at a predetermined frequency, and this drive frequency is determined by the pixel size of the CCD, the optical magnification of the CCD, and the feed rate of the sheet feed device 6. As an example, the CCD drive frequency is 123.4 nsec when the pixel size of the CCD is 18 .mu.m.times.13 .mu.m (sub-scan direction.times.main-scan direction), the optical magnification is 1:1, and the paper feed rate is 120 mm/sec. This CCD drive frequency is also used as a memory drive frequency when the CCD signal is stored in a memory 26. That is, when the optical image of the test patch 22 is projected onto the CCD 14, the CCD 14 outputs voltages which change in correspondence with the density of the optical image. These voltages from the individual pixels are line-sequentially extracted, subjected to A/D conversion of 8 bits, and stored in the memory circuit 26. In this case, it is more desirable to additionally provide a circuit for removing a reset signal contained in the CCD output.
FIG. 7 shows the state of the memory circuit 26 in which the CCD output is stored.
As shown in FIG. 7, the PWM lines of the test patch shown in FIG. 5 are reproduced in the memory. In the memory circuit 26 shown in FIG. 7, Pmax represents toner image portions of the PWM lines, and Pmin represents an interval between the lines of the toner image. In an image forming apparatus of an ideal system, the Pmax is equal to a CCD output at the time the maximum density of toner image portions of the test patch is detected, and the Pmin is equal to a CCD output at the time no toner density between toner image portions is detected at all. In an image forming apparatus of a practical system, however, the PWM lines are obtained as CCD outputs having smaller widths than Pmax-Pmin of the ideal system under the influence of spread of a latent image or scattering of the image during development and transfer. Note that in the CCD 55 used in the reader, the pixel size is 62.5 .mu.m.times.20 .mu.m, the optical magnification is 1:1, and the process speed is 160 mm/sec.
The above conventional example, however, has the following problems.
That is, in the formation of a latent image, a laser beam used as the exposing means forms a spot image of 45 .mu.m.times.75 .mu.m in the central portion of the photosensitive drum, whereas it forms blurred images at the end portions of the photosensitive drum as shown in FIG. 3C because of the reflection angle of the polygon mirror. Therefore, as shown in FIG. 8A, when the input signal to the exposing means is made uniform with respect to the thrust direction as the rotating direction of the photosensitive drum 1 as shown in FIG. 8B, a variation occurs in a toner density after development as shown in FIG. 8C.
In addition, independently of the development shown in FIGS. 8A to 8C, uneven coating of a photosensitive material layer and eccentricity of the rotating shaft of the photosensitive drum give rise to a nonuniform density variation in a toner density after development as shown in FIG. 9B with respect to a uniform input signal in the circumferential direction of the photosensitive drum as shown in FIG. 9A.
The inclination of the wire of the charger 2 with respect to the photosensitive drum 1, as shown in FIG. 10A, and the inclination of a developing sleeve 18 with respect to the photosensitive drum 1, as shown in FIG. 11A, bring about nonuniformities in distances between the photosensitive drum and these members, resulting in density gradients in the thrust direction of the photosensitive drum, as shown in FIGS. 10B and 11B, respectively.
Furthermore, the illuminating light sources 12a and 12b and the optical lens 13 are contaminated by toner scattering inside the image forming apparatus, and this decreases the reading power of the toner density of the test patch. Therefore, no highly precise image correction can be performed by using the patch on the photosensitive drum alone.